"William Gray of the Tropical Meteorology Project at Colorado State University has shown that hurricane activity waxes and wanes over 25 to 30 years. The 1910s and '20s were bad for hurricanes. Then came a period of calm, and another bad period in the 1940s and '50s. From the 1960s to 1995 was a period of calm."

"Robert Sheets, director of the National Hurricane Center in Miami from 1987 to 1995, agrees. He doesn't believe there's any solid evidence that Katrina was strengthened by global warming. 'Anything we've seen so far is not outside of what has occurred in the past,' he says."

"Christopher Landsea, a researcher meteorologist in the hurricane research division of the National Oceanic and Atmospheric Administration, says Katrina wasn't caused by global warming but is simply a part of the natural cycle of hurricane activity.

More from Landsea: "We've seen very busy times before, but the big difference is there's so many people living in hurricane alley. The coastal population is doubling roughly every 25 years from Texas to Carolina. That means the last time we were in a busy period there were many fewer people and less infrastructure in the way,' Landsea says."